MOUNT Gambier is sitting on unstable ground and living with the threat that a similar sinkhole to what opened up during last Thursday’s floods, could one day swallow a house or a street, according to a leading geologist.

When torrential rains dumped more than 100mm of rain on the city last week in a matter of hours, residents of Ramsay Avenue awoke to a sinkhole measuring 30 metres wide in Akuna Reserve having appeared overnight.

According to City Council’s operational services manager Darryl Sexton, this was not the first time the earth had collapsed in the area.

Mr Sexton said it had not only been luck which saw the sinkhole appear in a vacated park reserve rather than a backyard, however rather the ground’s “unstable” history was the reason it had become a reserve in the first place.

“It was once a car park, however when the earth collapsed for the first time the council converted it into a reserve for obvious reasons,” he said.

However, he said given the geological make-up of the land upon which Mount Gambier is built, there was no reason why it could not occur somewhere in the city a little less convenient – like underneath a home.

Award winning geologist Bernie Joyce – who has done extensive research into the volcanic history of the area – said due to the composition of the soil of the area, Mount Gambier was prone to more sinkholes collapsing.

“This area has been built on what was a volcanic region and in some places there was once lava flow – this means a lot of caves underground were formed that we do not yet know about,” he said.

“Secondly, the limestone which makes up the area is a very soft rock and often is dissolved and eroded by water, so a cave may have been formed by water a while ago.

“It could have just been plugged with volcanic ash – so in this instance it has reopened something that was already there.”

Mr Sexton said the hole would be filled in and re-grassed.

“It is a mystery why it keeps happening but I guess it is just nature in the making and it will not stop just because we live here,” he said.